This invention relates to wireless communications systems such as cellular telephone systems, and more particularly to cellular communications systems that provide asynchronous audio data to wireless subscribers.
Audio data may be transmitted to wireless subscribers in a variety of ways. One way involves the use of circuit switching. Wireless circuit switching is nearly identical to land line circuit switching because, in both cases, a communication path is established and dedicated for the duration of a call. An advantage of circuit switched communication is that the technology has been developed and refined for many years. However, a disadvantage of circuit switched combination is that the communication path is dedicated during the duration of the call. Dedication is inefficient when transmitting data because its transmission often involves short bursts of data occurring over very short periods of time. Therefore, the price paid for the call is for a much longer period of time than the time actually used to transmit data.
Another way of transmitting data over a wireless system involves the use of asynchronous packet switching. Packet switching involves breaking a data stream into a sequence of packets before they are transmitted. Each packet is transmitted by wireless system with a source address and a destination address, and then received, stored, and processed at a receiving terminal, such as a cellular telephone that corresponds to that destination address (i.e., subscriber ID). For example, voice can be transmitted over the internet and a radio channel using packet switching, in which the voice is converted to a series of packets. During the time interval that exists between the transmission of individual voice packets, other packets, voice or data, may be transmitted on the same channel. Therefore, in contrast to circuit switching, a radio channel or time slot of a radio channel (if a Time Division Multiple Access protocol is used) is not dedicated to a single subscriber over the duration of a call. Rather, two or more subscribers on a packet switched network can share network resources.
Wireless data is normally transmitted by using a Cellular Digital Packet Data ("CDPD") arrangement. CDPD takes advantage of the time intervals between successive packets of data on a single vacant channel and the pauses between sentences and syllables. Moreover, CDPD uses a channel-hopping technique that transmits short bursts of data between the end of one packet (i.e., voice or data) on one channel and the beginning of another packet on another channel. Wireless systems that use CDPD must install special equipment for transmitting and receiving CDPD, such as specialized base station and gateway equipment. An advantage of CDPD is that it accommodates a wide range of data services with a high degree of error protection comparable to many land line data networks.
CDPD has been used for reservation systems, electronic mail, vehicle tracking, and remote monitoring applications. These applications, however, demand a relatively small packet size--no more than about 4 or 5 kilobytes of data and are not ideally suited for the transmission of substantially continuous data, such as audio data.
In view of the foregoing, it is an object of the present invention to provide a method that uses asynchronous packet switching for applications that normally require substantially continuous transmission of data, such as audio data.
It is a more particular object of the invention to provide a system that uses CDPD for wireless transmission of audio data.